Input/Output (IO) for electronics and computing is typically described as the communication between some type of a processing system and the world outside of the subject processing system. The input of the IO refers to data/commands sent to the processing system from the outside world and the output of the IO refers to data/commands sent from the processing system to the outside world. The outside world simply refers to any entity not contained within the conceptual boundaries of the subject processing system. A processing system may be as small as a portion of a single chip or as large as networked group of computers, where the designation of the “processing system” is relative to what system is being designed. For instance, a computer integration specialist may look at each computer on a system as the “processing system,” while an integrated circuit chip designer may look to an individual integrated circuit chip, or even individual subsystems within a particular integrated circuit as the “processing system” such that the chip or chip subsystem operates as its own “processing system” with IO to the outside world being IO to entities outside of the chip or even other subsystems contained on the same chip if the “processing system” is an individual subsystem of a particular chip.
An IO interface is necessary for the processor system to interact with the outside world. The IO interface provides the necessary logic to interpret addresses/commands issued by and/or sent to the processing system. A typical IO interface will implement some form of “handshaking” with the device/entity of the world outside of the processing system using commands such as busy, ready, and/or wait. Once “handshaking” to establish a connection is complete, a typical IO interface permits communication between the processing system and the world outside of the processing system. If different data formats are being exchanged, the IO interface typically performs the necessary conversion between the data formats to permit communication.
A typical integrated circuit chip is made up of individual electrical elements and typically includes active elements such as one or more transistor devices. Typical operation of a transistor requires connection of the transistors either directly or through other electrical devices to a high and a low output of a voltage supply. Typically, the high output of the voltage supply is identified as the high voltage supply and the low output of the voltage supply is identified as the low voltage supply. For an integrated circuit based on Metal-Oxide-Semiconductor Field-Effect Transistors (MOSFETs), it is common to refer to the high supply voltage as VDD (or VDDIO when a separate power supply is available for the IO interface) and to refer to the low supply voltage as VSS. Many if not most, times VSS is also referred to and defined as the electrical ground for a circuit. Typical operation of a MOSFET device involves the application of high (VDD) and or low (VSS) supply voltages to the gate, source, and/or drain either directly or through other electrical circuit devices. MOSFET devices are typically divided into two major subdivisions, PMOS devices and NMOS devices which have related, but different operational characteristics. While initially tied to MOSFET based circuits, the use of VDD and VSS to represent the high and low voltage outputs, respectively, has become common usage even for non-MOSFET based circuits.